The present invention relates to a rubber or resin composition. More particularly, the present invention relates to a rubber or resin composition which is capable of suppressing to the minimum the decomposition or degeneration, i.e., the deterioration of rubber or a thermoplastic resin which is caused by the influence of the heat applied to the rubber or the thermoplastic resin during processing or the light during exposure to the open air when the rubber or the thermoplastic resin is colored with an iron oxide pigment.
Thermoplastic resins such as vinyl chloride resin, polyester resin, polypropylene resin, polyethylene resin, polyamide resin, polycarbonate resin and ABS (acrylonitryl-butadiene-styrene copolymer) resin, natural rubber; and synthetic rubber such as polyisoprene rubber, styrene-butadiene rubber, ethylene-propylenediene rubber, acrylonitryl-butadiene rubber and silicone rubber are colored with a coloring agent such as a known iron oxide pigment and widely used as a molding material.
Thermoplastic resins, even if they are not colored with an iron oxide pigment, are decomposed and degenerated by the heat applied thereto during processing or the sun light striking them during exposure to the open air after they are molded into products. If a coloring agent such as an iron oxide pigment is mixed with the rubber or thermoplastic resin, the deterioration of the rubber or the thermoplastic resin is greatly accelerated due to the heat-deterioration acceleration of the coloring agent or the change acceleration of a weather-resistant material.
Such deterioration is remarkable in a resin containing chlorine such as polyvinyl chloride, chlorinated polyethylene, chlorinated polypropylene and polyvinylidene chloride. Especially, this is a great problem in polyvinyl chloride which is generally used.
The deterioration of rubber of a resin will be explained in the following while citing a polyvinyl chloride resin which is typical of chlorine-containing resins.
Such deterioration is generated because a part of chlorine bonding in a polyvinyl chloride resin is decomposed by heat at 100 to 200.degree. C. or by light and produces hydrogen chloride, thereby forming a polyene structure having a double bonding. Further hydrogen chloride produced secondarily acts on the resin itself, thereby cutting a part of C--C bonding of a polymer or giving rise to a cross-linking reaction as a chain reaction.
It is generally known that in order to suppress such deterioration, a neutralizer such as a lead compound and metallic soap for neutralizing hydrogen chloride produced by decomposition or a resin stabilizer such as an organic tin compound and an epoxy compound which is effective for suppressing the generation of a double bonding, is added to a polyvinyl chloride resin for an ordinary thermoforming material even when no iron oxide pigment is used.
As a coloring agent for a polyvinyl chloride resin, iron oxide pigments as inorganic pigments having excellent heat resistance and weather resistance, are conventionally widely used. As is known, since metals such as Mn, Fe, Co, Ni, Cu and Zn accelerates dehydroclorination, when an iron oxide pigment is mixed with a polyvinyl chloride resin as a coloring agent, the deterioration of the resin is greatly accelerated.
An iron oxide pigment which shows excellent deterioration resistance is therefore strongly demanded.
Various Methods of improving the deterioration resistance of an iron oxide pigment have conventionally been investigated. For example, a method of forming a continuous fine silica coating film on the surfaces of iron oxide particles (Japanese Patent Publication (KOKOKU) No. 54-7292 (1979)), a method of coating the surfaces of the iron oxide particles with glass-like Na-- and/or K--Si materials (Japanese Patent Publication (KOKOKU) No. 53-11537 (1978)), and a method of precipitating SiO.sub.2 on the surfaces of the iron oxide particles and further precipitating aluminum hydroxide on the precipitated SiO.sub.2 (Japanese Patent Application Laid-Open (KOKAI) No. 53-36538 (1978)) are known.
Although an iron oxide pigment which shows excellent deterioration resistance is now in the strongest demand, the above-described known iron oxide pigments cannot be said to have a sufficient deterioration resistance, as will be shown in a later-described comparative examples.
On the other hand, magnetite particles which contain a silicon component therewithin and also have a silicon component exposed to the surfaces, and which have a small residual magnetization, high electric resistance, and excellent operability and fluidity are proposed in Japanese Patent Application Laid-Open (KOKAI) No. 5-213620 (1993). The object of the invention of Japanese Patent Application Laid-Open (KOKAI) No. 5-213620 (1993) is to provide magnetite particles for a magnetic toner for an electrostatic copying machine, and the specification thereof is completely silent about an iron oxide pigment which is capable of suppressing the deterioration such as decomposition and degeneration of a rubber or thermoplastic resin, that is, which shows excellent deterioration resistance. The specification is also completely silent about rubber or resin composition which is composed of the iron oxide pigment and a rubber or thermoplastic resin and which shows excellent deterioration resistance.
Accordingly, it is a technical problem to be solved in the present invention to improve the deterioration resistance of a rubber or resin composition containing an iron oxide pigment, that cannot be realized or expected in the prior art.
As a result of studies undertaken by the present inventors, it has been found that by adding to 90 to 99.9 parts by weight of rubber or thermoplastic resin to 0.1 to 10.0 parts by weight of specific iron oxide particles composed of iron oxide core which contains 0.21 to 2.14 wt % of silicon (calculated as SiO.sub.2) and a precipitate composed of silicon of 0.01 to 10.0 wt % (calculated as SiO.sub.2) which adheres to the surfaces of the iron oxide core, and having a sphere ratio (maximum diameter/minimum diameter) of 0.7, the obtained rubber or resin composition has, when heated at 190.degree. C. for 90 minutes, an area percentage of the part the color of which is changed of not more than 25%, that is, such rubber or resin composition has excellent deterioration resistance. The present invention has been achieved on the basis of this finding.